I have an HTC One and used the stock supplied ROM for a few months before deciding to take the plunge into "Root and Custom ROM installation" world. I rooted the device, installed Clockwork Mod Recovery and a custom ROM (Android Revolution from XDA developers which is based on 4.3.3 Jellybean I think.)
There were a few nervous pauses through both the rooting and the ROM install process and a couple of "OMG I've bricked it" moments when It would take longer to reboot than I expected, but apart from that the whole process was fairly painless. Of course I neglected to back up first and lost data (not SD card, that isn't touched, but locally stored contacts etc. were wiped..) but that, to my mind, is the sort of mistake you should only make once.
I hugely prefer Android Revolution to the stock ROM. My phone is quicker, more responsive, has additional features and gives me a much deeper level of "ability to fiddle" that is available on stock. My battery time (standby) has nearly trebled and I seem to get a much more reliable 3g connection than before (maybe not ROM related..unsure).
As it is included in the ROM (also available standalone I think) I now have access to the Sense toolbox which allows you to further customize the phone and fiddle with things you were never really intended to fiddle with (which I like and is partly why I rooted it in the first place)
Provided you have a slightly more than basic understanding of Android and your phone hardware, rooting and Installing custom ROMs is not difficult. The whole process can be completed in under an hour, and the biggest benefit to me it I now have the confidence to know that I can mess with these things (fairly) safely.
But please, do make a backup of locally stored data, and also created a backup of your current install via your chosedn custom recovery app, just in case you do have reason to need to restore. Looking forward to Kit Kat now..